Thermal monitoring integrated along underground power cables is of high interest for electricity, gas and liquid transport infrastructure management. Failure scenarios like thermal runaway due to soil drying, junction failures, leaks or external mechanical actions can cause heavy damages and injure people. Any unsolved failure can also become very expensive. Moreover, lifetime estimation is of great interest to optimize asset management and forecast investments. Conventional fibre optic temperature monitoring is an efficient tool to tackle those issues but is reserved for strategic infrastructure because of the high cost of the existing solutions (based on Raman and Brillouin scatterings). The scope of application of the invention can also be extended to fire and presence detection.
Sensing systems based on interference pattern disturbance have been widely studied, which allowed the realization of statistical mode sensors as described by Spillman, W., Kline, B., Maurice, L., & Fuhr, P., in article “Statistical-mode sensor for fiber optic vibration sensing uses”, in Applied optics, 28(15), 3166-76., 1979 and more recently few-mode fiber sensors as disclosed by Safaai-Jazi, A., & McKeeman, J. C., in article “Synthesis of Intensity Patterns in Few-Mode Optical Fibers”, Journal of Lightwave Technology, 9(SEPTEMBER), pp. 1047-1052, 1991 and by Kumar, A., Goel, N. K., & Varshney, R. K., in “Studies on a Few-Mode Fiber-Optic Strain Sensor Based on LP 01 -LP 02 Mode Interference”, Journal of Lightwave Technology, 19(3), pp. 358-362, 2001.
This approach was mainly applied to vibration and strain sensing. Temperature sensing based on pattern recognition is rather tricky for three main reasons. Firstly only temperature variation can be measured by usual methods instead of absolute temperature. The temperature is therefore obtained by an integration operation (versus time), which causes error drifts. Secondly discrimination between a heating phase (positive temperature variation) and a cooling phase (negative temperature variation) is not straightforward. Thirdly the discrimination between temperature and vibration effects must be efficient.